The focal point of my efforts will be an analysis of the developmental relationships within the lympho-myeloid complex. Primary experimental procedures will involve the reciprocal transplantation of primitive tissue anlagen between early embryos of chromosomally-labeled Leopard frogs (Rana pipiens). The experiments are designed to elucidate the embryonic sources and migratory pathways of the hematopoietic stem cells which populate the thymus from early embryogenesis through metamorphosis. Included is an analysis of the absolute necessity of the thymus to the developing immune response, both as it relates to hematopoiesis in general and its specific role in the initiation of alternate pathways of lymphopoiesis. Finally, functional attributes of mature immunocompetent cells will be correlated with their sites of embryonic origin.